Field of the Invention
This invention relates generally to short circuit detection in electric machines and, more particularly, to a technique for identifying turn-to-turn and phase-to-phase winding shorts in an electric motor at standstill by injecting a current signal into the virtual d-axis of the motor while controlling q-axis current to zero, measuring feedback current from the motor, and determining that a short circuit is present in the winding if winding inductance has decreased as indicated by increased feedback current harmonics.
Discussion of the Related Art
Known electric motors include permanent magnet electric motors and other induction motors that transform electric power to mechanical torque. Permanent magnet electric motors may be multiphase permanent magnet (PM) electric motors that include permanent magnets as part of a rotor core and aligned longitudinally with an axis of rotation. Known stators include an annular stator core and a plurality of electrical windings. Stator cores commonly include a plurality of radial inwardly projecting tooth elements that are parallel to a longitudinal axis of the electric motor and define an inner circumference of the stator. Contiguous radial inwardly projecting tooth elements form radially-oriented longitudinal slots.
Electrical windings are fabricated from strands of suitable conductive material, e.g., copper or aluminum, and are woven or otherwise arranged into coil groups that are inserted into the radially-oriented slots between the tooth elements. Electrical windings are arranged electrically in series in circular fashion around the slots of the stator core, with each electrical winding associated with a single phase of the electric motor. Each coil group of the electrical windings provides a single pole of a single phase of motor operation. The quantity of radially-oriented slots in the stator core is determined based upon the quantity of phases and poles of the electrical wiring windings for the electric motor. Thus, a three-phase, two-pole motor will have electrical windings that are configured as six coil groups. Current flow through the electrical windings is used to generate rotating magnetic fields that act on the rotor to induce torque on a shaft of the rotor.
Known rotors for permanent magnet electric motors include a rotor core attached to a rotating shaft that defines an axis of rotation, and have a plurality of rotor magnets positioned around the circumference near an outer surface of the rotor core, with each rotor magnet aligned longitudinally with the axis of rotation.
Known electric motors include an air gap between tooth elements of the stator and an outer surface of the rotor. The air gap is a design feature that physically separates the rotor and stator to accommodate manufacturing tolerances and facilitate assembly. The air gap is preferably minimized, as an increased air gap correlates to reduced magnetic flux and associated reduced output torque of the electric motor.
When electric current flows through the stator windings, a magnetic field is induced along the electrical windings to act upon the rotor magnets of the rotor element. The magnetic field induces torque on the rotating shaft of the rotor. When the magnetic field induces sufficient torque to overcome bearing friction and any induced torque load on the shaft, the rotor rotates the shaft.
Each of the coils in the stator winding contains several “turns” of insulated conductor. The rotor winding is subjected to a variety of loads, stresses and environmental factors—including centrifugal forces, thermal expansion/contraction, vibration, high voltages and high temperatures—which can lead to deterioration and breakdown of the insulation. When the insulation breaks down, a short circuit between adjacent turns of the winding, or turn-to-turn short circuit, results. In other circumstances, rotor windings can develop a phase-to-phase short circuit.
Winding short circuit faults, if present during operation, can cause motor damage or failure. It is therefore desirable to detect short circuits before motor operation, when the motor is at a standstill.